The present invention is generally directed to an implantable stimulation device for monitoring the progression or regression of heart disease and modifying stimulation in response thereof. The present invention is more particularly directed to such a device which monitors one or more physiological parameters of a patient, over an extended time period, indicative of the progression or regression of congestive heart failure.
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body""s tissues. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract.
Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow may become leaky, allowing regurgitation or back flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result.
Not all CHF patients suffer debilitating symptoms immediately.
Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
As CHF progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart""s oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output.
CHF has been classified by the New York Heart Association (NYHA). Their classification of CHF corresponds to four stages of progressively worsening symptoms and exercise capacity from Class I to Class IV. Class I corresponds to no limitation wherein ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity wherein such patients are comfortable at rest, but where ordinary physical activity results in fatigue, shortness of breath, palpitations, or angina. Class III corresponds to a marked limitation of physical activity wherein, although patients are comfortable at rest, less than ordinary activity will lead to symptoms. Lastly, Class IV corresponds to inability to carry on any physical activity without discomfort, wherein symptoms of CHF are present even at rest and where with any physical activity, increased discomfort is experienced.
Current standard treatment for heart failure is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Only an option in 1 out of 200 cases, heart transplantation is also available. Other cardiac surgery is also indicated for only a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, patients in NYHA Classes III or IV, who are still refractory to drug therapy, have a poor prognosis and limited exercise tolerance. Cardiac pacing has been proposed as a new primary treatment for patients with drug-refractory CHF.
By tracking the progression or regression of CHF more closely, treatments could be administered more effectively. Commonly, patients adapt their lifestyle and activities to their physical condition. The activity level of the patients with NYHA Class III or IV would be much lower than that of the patients with NYHA Class I or II. The change in lifestyle or activity level, due to the patient""s heart condition, will be reflected by activity and respiration physiological parameters.
The present invention addresses the issues of tracking CHF progression or regression to enhance the administration of therapies and to enable the monitoring of the effectiveness of such therapies.
The present invention provides an implantable cardiac device for detecting a progression or regression in heart disease such as congestive heart failure. An activity sensor generates raw sensor signals indicative of the patient""s activity level. Degradation or improvement of the patient""s activity level over a predetermined extended time corresponds to an indication of the progression or regression of the heart disease. A processor processes the raw sensor signals over the predetermined extended time and determines relative changes in activity levels. A memory having a data storage area stores the activity levels during the predetermined extended time and a telemetry circuit transmits the activity levels to an external monitor for display.
The activity measurements are taken at frequent intervals. The processor at time intervals such as once per day stores selected activity levels in the memory. The selected activity levels stored may, for example, represent a maximum change in activity level recorded during the last 24 hours.
Similarly, a respiration sensor generates raw sensor signals indicative of the patient""s respiration. Degradation or improvement of the patient""s respiration levels over the predetermined extended time further corresponds to an indication of progression or regression of the heart disease. The processor processes the raw respiration sensor signals to determine respiration levels. The respiration levels are preferably frequently determined for updating previously stored respiration levels in the memory. At the end of the time interval, for example every 24 hours, the processor samples selected respiration levels and stores the selected respiration levels in the memory. The selected respiration levels may represent maximum and minimum values corresponding to selected respiration parameters.
At the end of the predetermined extended time which may be, for example, 24 weeks, the activity and respiration parameters for each day of the last 24 weeks are available in the memory to be transmitted to the external monitor for display. The memory is preferably configured to be a circulating memory so that at any time the selected daily stored activity and respiration levels over the last 24 weeks are available for transmission to the external monitor for display.
In accordance with further aspects of the present invention, the processor maintains a histogram of activity and respiration levels. This histogram is updated at frequent intervals such as every 30 seconds. At the end of a time period as, for example, at the end of each week, the processor derives from the histogram a set of histogram values which are then stored in the memory. The histogram values stored each week may be maintained by the memory over the predetermined extended time of, for example, 24 weeks.
The activity parameters stored in the memory at the end of the time intervals as, for example, each day, in accordance with the present invention, include active time count which represents the longest duration of the activity being larger than the long-term activity average level, maximum activity variance level representing the maximum activity variance recorded during the last 24 hours, maximum activity taken from the activity histogram, and median and mode activity variance taken from an activity variance histogram. The respiration levels which are stored in the memory after each 24-hour period, in accordance with further aspects of the present invention, may be the maximum values for respiration rate, tidal volume and minute ventilation during sustained exercise episodes, base-line respiration rate, tidal volume and minute ventilation during normal respiration in resting condition, respiration rate, tidal volume and minute ventilation during abnormal respiration such as cyclic breathing, hyper and hypo ventilation periods, a single cyclic breathing period, total duration, total number of cyclic breathing, total time in cyclic breathing, and total time in normal sleep. The respiration levels, in accordance with the present invention, are derived from impedance levels made by the respiration sensor.
The histograms maintained in the memory by the processor preferably comprise three histograms. One histogram provides an historical record of recent activity levels of the patient. Another histogram provides an historical record of the activity variance of the patient and a third histogram provides an historical record of the difference between short-term respiration levels and long-term respiration levels of the patient. After the histogram values are determined by the processor, the histograms are cleared. As a result, at any time, the memory will contain histogram values taken each week over a 24-week period.
The foregoing provides a comprehensive history of the patient""s activity and respiration levels over an extended period of time. While the recorded levels made available for display are comprehensive in nature, only limited memory space is required for holding the most recent parameters for an extended period of 24 weeks. Upon display of the recorded levels, the progression or regression of the heart disease may be readily determined.
In accordance with a further aspect of the present invention, when the device is a cardiac rhythm management device for delivering therapy, such as pacing therapy to the patient""s heart, the device itself may adjust therapy responsive to the determined physiological parameter levels. The therapy adjustment may take the form, for example, of pacing rate adjustments to assist the patient in breathing or in the removal of fluid from the lungs.
In accordance with a further aspect of the present invention, the respiration measurements are taken during different patient activity conditions. To that end, when it is time to take and update respiration levels, the respiration and activity measurements stored are determined by the patient activity condition.